1. Field of the Invention
The present invention relates to a light emitting device having a light emitting element, and a manufacturing method thereof.
2. Description of the Related Art
A light emitting element having a light emitting material interposed between a pair of electrodes (an anode and a cathode), which is thin, light, and capable of fast response, is expected to be applied to a next-generation flat panel display. Furthermore, a light-emitting device in which light emitting elements are arranged in a matrix is said to be superior to a conventional liquid crystal display device in that it has wider view angle and higher visibility.
A simple matrix mode and an active matrix mode can be given as driving methods of a light emitting device, and the active matrix mode using an active element such as a thin film transistor (TFT) is adopted in many cases for high-definition panels (for example, a panel having more pixels than QVGA).
In the case of an active matrix light emitting device, a TFT formed in each pixel in a pixel portion is electrically connected to a light emitting element, and the light emitting element has a structure in which a light emitting material is interposed between one electrode (hereinafter referred to as a first electrode) connected to the TFT and the other electrode (hereinafter referred to as a second electrode). The second electrode of the light emitting element is formed of one conductive film which is common to all light emitting elements in the pixel portion.
However, as the number of pixels is increased to realize a higher definition panel, the second electrode formed of the conductive film which is common to all pixels comes to have more voltage drop due to material resistance of the electrode or the like, and a voltage applied from the second electrode varies among light emitting elements in respective pixels. Since luminance of a light emitting element depends on a current amount, the voltage variation leads to variation in luminance of light emitting elements.
With respect to this problem, a method in which a wiring structure and a wiring layout are optimized so that a driving voltage or a current is supplied stably to light emitting elements of respective pixels has been employed. (For example, Patent Document 1: Japanese Patent Laid-Open No. 2004-102245)
However, in the case of the above-mentioned Patent Document 1, in order to reduce voltage drop due to the resistance of a connection wiring of a second electrode, the connection wiring for the second electrode is formed at the periphery of a pixel portion, with a wider line width than that of a power supply wiring for light emission, which is not favorable when it is desired to decrease a non-active area of the panel (when it is desired to reduce the frame size).